Devices of this type are used for coating glass plates or plastic sheets with electrically conductive and/or heat-reflecting films. Generally, a base film is first applied to improve the adhesiveness of the substrate to be coated. This adhesive film can be formed of a metal oxide or a metal sulfide. Next a heat reflecting or electrically conductive film of gold, silver or copper is applied on top of the adhesive film. Finally, another metal oxide or metal sulfide film is applied for protection. A reliable film sequence for coating of silicate glass plates consists, e.g., of a tin dioxide film as the adhesive film, a silver film as the reflecting film, a thin metal oxide protective film, and finally an anti-reflection film of tin dioxide.
Vacuum coating devices work very economically if they operate according to the principle of magnetic field aided reactive cathode sputtering. To apply a metal oxide film with such a sputtering device, a metal target is used as a cathode. Atoms of the metal are knocked from the target in order to react with oxygen gas which is fed into the coating chamber. The resultant metal oxide is deposited on the substrate to form the desired film.
Vacuum coating devices, which operate according to the principle of cathode sputtering, usually contain additional devices between the cathode and the substrate. For example, anode electrodes are placed close to the cathodes to spread and stabilize the plasma cloud. For this purpose, an electric potential is applied to the anode electrode which is positive in relation to the electric potential of the cathode and to the potential of the metal housing of the vacuum chamber. Known anodes have the shape of round sections or tubes. Shielding plates or screens placed above the substrate are another example. The function of the screens is primarily to let only the high energy main beam of the sputtered particles reach the substrate surface by screening out the lower energy lateral portions of the particle beams. To achieve this result, a potential equal to or more positive than the ground potential of the coating housing is applied to these screens.
During operation of the installation, deposits of the coating material, which grow in thickness in the course of time, are formed on exposed surfaces within the vacuum chamber. Great mechanical stresses develop in these deposit films or between these deposit films and the metal base. These mechanical stresses can reach especially high values due to the temperature changes to which the coating installation is exposed, particularly during start up and shut down, since the metal components and the nonmetallic deposits have substantially different thermal expansion coefficients. Since the deposit films consist of nonductile, i.e., brittle material, the stresses cannot be reduced by plastic deformation. Over time as the deposits become increasingly thicker and the mechanical stresses become increasingly greater, the intrinsic strength of these deposit films is exceeded. Under the effect of these stresses, parts of the deposited films chip off from the base and fall or jump through the screen opening onto the substrate. As a result, coating defects occur on the substrate. To avoid such coating defects, the coating process must be interrupted at regular intervals to remove deposits from the affected components or to replace the affected components with clean components. These necessary interruptions drastically limit the productivity of the coating installation.